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Old 01-11-2022, 01:23 PM
Steve C. Steve C. is offline
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This from Jim Butler. He used the 4.250 stroke crank and 6.8 rods, along with his custom JBP Ross pistons. It increases the displacement of the 455 engine. At the same time, the better rod angle, the rod-to-stroke ratio of 1.6:1, and the raised pin location in these custom pistons relieve the stress on the block. The higher pin pushes up on the piston instead of out on the skirt and results in less friction.


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Old 01-11-2022, 02:04 PM
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At an equal crankshaft height, an engine with long connecting rods can mount a shorter and lighter piston, reducing the alternating masses to the advantage of reliability and decreased component wear.
Furthermore, long connecting rods also have reduced angles during movement, generating less lateral thrust and friction (less wear) on the cylinder walls. This is why road engines favour smaller pistons at the expense of longer connecting rods.

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Old 01-11-2022, 03:33 PM
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Originally Posted by hurryinhoosier62 View Post
Not really. Longer rods increase the side loading on the major thrust surface of the cylinder bore.
Can you explain why / what about a longer rod causes that?

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Old 01-11-2022, 04:17 PM
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Originally Posted by takid455 View Post
Figured it was something along the above.

Not to derail the thread too much, given the choice, would you run a 4.21 crank & have it offset ground to deck the pistons with 73/4 SD rods or 4.25 w/ new rods? SD Rods are acquired as with SRP forged pistons. Crank is needed. Application/ purpose - fun street. mid -upper 400hp
4.21 stroke with SD rods will easily make 400 hp. Depending on the heads you have and going with right cam. On stock Pontiac rod Journal size. So no need to off set grind the crank. So if you already have the 4.21 stroke crank and SD rods and only looking for 400 hp, you can accomplish that like falling out of a tree.

As far as deck goes, it’s just a matter of having the block squared. Which should be no problem with a stock block. Also cam choice would depend on weather it’s a 4 speed car or automatic. With automatic convertor stall comes into play on cam choice, and the amount of slippage your willing to live with. So a cam that utilizes say 3500 stall still makes a nice street cruiser.

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Old 01-11-2022, 04:43 PM
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Currently, I do not have a crank. Have everything else. Part of the reason of asking the original question.

Heads are 16 rdpt
Cam. HR version of the HO racing HC03 (240/250) @50.
Stick with DN 4+1. 273 rear. 27 or 26" tire

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Old 01-11-2022, 04:51 PM
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The longer 6.800 rod, was all the rage, many different scenarios, one was it dwelled at top dead center longer, to take advantage of our very poor head flow. The other scenario is it made the rod to stroke ratio better. So it’s all depends on which scenario you believe in. in actuality it doesn’t care what length rod you use. The best engine builder will tell you that, and I believe that and here’s why, when that rage was going on, I was Joneson to go with the longer rod, so after year or better bracket racing, it was time to refresh the motor. So I went with the longer 6.8 rods. On my 462, thinking I was going to pick up hp, I was all excited, when I dyno the motor I was very disappointed it pick up ZERO hp. Was the 6.8 rod a gimmick to get you to buy those rods, I was running SD rods and went to Crower 6.8 rod. Gimmick I don’t really think so, one thing I’ll say, I was able to put 300 runs on that motor, with security of not having a rod failure. The SD rods were good rods, but rod bolt failure was a issue if you didn’t install new rod bolts. Which at time getting harder to get and not cheap, to have installed and check for resizing. Just my experiences. Take it for what ever it’s worth, I think most of all the rpm level was really the saving grace. I only shifted at 5500 rpms. Making 625-650 hp.

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Old 01-11-2022, 05:12 PM
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Originally Posted by takid455 View Post
Currently, I do not have a crank. Have everything else. Part of the reason of asking the original question.

Heads are 16 rdpt
Cam. HR version of the HO racing HC03 (240/250) @50.
Stick with DN 4+1. 273 rear. 27 or 26" tire

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Stick, that’s a good thing, because with HO03 cam and automatic your stall would be up there closer to 3800- 4000 and not really very street able for just a Cruiser.
It’s a great cam I ran it with #96 d-port heads if your just going to be cruising with it should be fine. The only problem I see is the 273 gear, it’s going want to make peak torque around 3800+. Definitely much better with at least 3.73 gear. It’ll be a dog on the bottom end, but have all kinds of top end. As far as crank goes, and trying to used the SD rods which have stock Pontiac rod Journal your going to be limited. On crank choice. Even buying aftermarket crank. Not much choice on cranks with stock rod journals out.

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Old 01-11-2022, 05:15 PM
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Just always thought it strange they went from 3.75 to 4.00 but then 4.21. Why not 4.25 from the factory?
I’m thinking it something to do with running one common rod.

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Old 01-11-2022, 05:21 PM
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400 hp with that cam be no problem whatsoever. It’s just getting the whole combination right, the right gear is critical. In how it feels 0 to 60. Especially depending on the weight of your car, if it’s a havey car it’s definitely going to want more then a 273 gear.

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Old 01-11-2022, 05:27 PM
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Originally Posted by grandam1979 View Post
I’m thinking it something to do with running one common rod.
There should have been no problem running the same rod with either the 4.21" or 4.25" stroke. The 4.21" needed a 1.47" compression height piston the 4.25" needed a 1.45" compression height piston.

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Old 01-11-2022, 06:29 PM
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Originally Posted by Stan Weiss View Post
Can you explain why / what about a longer rod causes that?

Stan
Simple physics, Stan. As a piston travels up the bore on a long rod, the primary thrust face of the piston “unloads” on the primary thrust face of the cylinder. The longer the rod, the more the unloading occurs. In low rpm engines it is manageable, but once this starts in a high rpm engine it can be catastrophic. This is WHY shorter and lighter pistons are used in coordination with long rod/long stroke combinations. Ever seen a long rod, long stroke F1 engine or IMS engine? What works in street and drag racing engines doesn’t work in forms of competition where high rpms (8,000 or higher) for extended periods are very common. Back in the “old days” of NASCAR long rod/short stroke engines were preferred on short tracks while longer stroke longer rod engines were preferred on super speedways (back then Daytona, Talladega, Michigan and Pocono). WHY? The long rod short stroke combination gave great torque out of the turns onto the straightaways. On a bad day, you might see 7,000 rpm a few times( remember, we are taking about BBCs, Elephant Hemis and FE Fords/ Boss 385 big blocks). On the super speedways engine speeds in excess of 7,000 RPM were common as were cylinder wall collapses. WHY? The longer stroke/longer rod combinations were unloading into the primary thrust face of the cylinder walls. At a continuous 7,000+ rpm it doesn’t take all that long. Today, NASCAR mandates a 3.310 maximum stroke with a 6.200 maximum rod length(?...haven’t seen a NASCAR rule book in a while); a relatively short stroke with a relative short rod( the opposite of our beloved Pontiacs). You can learn a lot listening to old NASCAR/ARCA and IMS/USAC engine builders. Consider this: in 1970 Plymouth Superbirds and Dodge Daytona were running Hemis with 7.200 c-to-c connecting rods on the super speedways(3.750 stroke). That is 3/8 of an inch LONGER than we typically use in modern Pontiac stroker builds today. Now, try to find a NASCAR Hemi block that hasn’t been sleeved due to a cylinder wall collapsing...

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Old 01-11-2022, 11:53 PM
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400 hp with that cam be no problem whatsoever. It’s just getting the whole combination right, the right gear is critical. In how it feels 0 to 60. Especially depending on the weight of your car, if it’s a havey car it’s definitely going to want more then a 273 gear.
The 2.73 & the 1:1 5th are the cruising gear. 1st gear in that trans is a 3.27. Had a 4.10 1st which made a harsh 1-2 shift. 2nd gear was 2.20 ish . The car has been together for 13k miles with the FT HC03 & was pretty happy. I know the gear change will hurt some acceleration, but I'd rather the stronger top end than go to a lower rear. Car is typical 2nd gen w/ no weight saving measures. Doing some work on it & juggling the idea of changing the crank / internals because we can. The SD rods have been resized with ARP bolts connected to SRP forged pistons. Part of me says just go all new for piece of mind. The other part says, save what you can & upgrade the crank. I do like the shorter piston theory as long as it can remain stable. Realistically for the use of this car, either selection will be fine. It'll be street with occasional 6k blast ripping through the turns. That's what the old cam would hit.

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Old 01-12-2022, 12:46 AM
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The 2.73 & the 1:1 5th are the cruising gear. 1st gear in that trans is a 3.27. Had a 4.10 1st which made a harsh 1-2 shift. 2nd gear was 2.20 ish . The car has been together for 13k miles with the FT HC03 & was pretty happy. I know the gear change will hurt some acceleration, but I'd rather the stronger top end than go to a lower rear. Car is typical 2nd gen w/ no weight saving measures. Doing some work on it & juggling the idea of changing the crank / internals because we can. The SD rods have been resized with ARP bolts connected to SRP forged pistons. Part of me says just go all new for piece of mind. The other part says, save what you can & upgrade the crank. I do like the shorter piston theory as long as it can remain stable. Realistically for the use of this car, either selection will be fine. It'll be street with occasional 6k blast ripping through the turns. That's what the old cam would hit.
Didn’t realize it was a five speed, i’m always taking Turbo 400 or four speed. Now I understand were your coming from, you seem to have it cover also didn’t realize it’s a motor you already have in the car. Sounds like your pretty happy with the power. So now I understand 273 gear. If you go 4.25 stroke know you realize also need a piston change. The thing you haft to think about is whether or not you want to make a rod change. Because no matter what you do, the SD rods still have stock Pontiac ride journal size. Most if not all 4.25 stroke are BBC size 2.200. Question is, if you make change over to 4.250 stroke is it really going to change anything, I doubt there’d be any notice able gain, but just wanting to update everything because of number of miles on combo. I’m sure you’ve already thought of that. Personally I run 4.250 stroke steel crank with stock Pontiac Rod length, BBC rod Journal, only because of what I do, don’t trust Pontiac stock cast crank. I definitely like stock Pontiac rod length better. Rpm is the killer on cast cranks. My suggestion is to go with a steel crank.

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Old 01-12-2022, 03:45 AM
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Quote:
Originally Posted by takid455 View Post
The 2.73 & the 1:1 5th are the cruising gear. 1st gear in that trans is a 3.27. Had a 4.10 1st which made a harsh 1-2 shift. 2nd gear was 2.20 ish . The car has been together for 13k miles with the FT HC03 & was pretty happy. I know the gear change will hurt some acceleration, but I'd rather the stronger top end than go to a lower rear. Car is typical 2nd gen w/ no weight saving measures. Doing some work on it & juggling the idea of changing the crank / internals because we can. The SD rods have been resized with ARP bolts connected to SRP forged pistons. Part of me says just go all new for piece of mind. The other part says, save what you can & upgrade the crank. I do like the shorter piston theory as long as it can remain stable. Realistically for the use of this car, either selection will be fine. It'll be street with occasional 6k blast ripping through the turns. That's what the old cam would hit.
Butler has of the self Ross pistons for 6.7 rods. They are BBC and would go with a steel crank with 2.20 rod journals.
You could even go with a 4.375 forged crank and 6.635 (near Pontiac) BBC rods and custom piston.
You would feel the difference in tq with your heavy car.
All kinds of things you can do. But if it involves Pontiac SD rods you are stuck with 4.21 stroke,3.75 or 4". And thats not so bad.

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Old 01-12-2022, 11:03 AM
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The reason this whole combo started was before there were aftermarket cranks you could offset grind a 4.21 " 455 crank to 4.25" using smaller BBC journals and then a common aftermarket forged BBC rod at 6.8 that was close to Pontiac length. Forged Pontiac rods were rare back then also unless you went with Carillo or Crower billet which were pricey(over $1K in the 80s-90s). Or use Butler/Banry Moravitz heat treated 58-62 rods.

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  #36  
Old 01-12-2022, 11:56 AM
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Palbykins NA 473 cid engine used a 2.10" SB Chevy Rod Journal on his cast iron 455 crank. Custom Rods and Pistons. Believe the Pistons were Ross.

So you can go smaller than a 2.20 rod bearing.

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Old 01-12-2022, 12:13 PM
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Quote:
Longer rods increase the side loading on the major thrust surface of the cylinder bore.

It's the R/S ratio that affects the side loading. It increases with SMALLER R/S ratio. (R/S - Rod length to Stroke length)
Higher R/S ratio makes for higher RPM with less loading.


The R/S of a 455 with stock stroke of 4.21 is 1.57, pretty bad.
The R/S of a 455 with same stroke and 6.8" rod length is R/S of 1.62, still probably bad but better than 1.57.
The R/S of a 4.151 bore and 4.25 stroke with 6.8 rod length is R/S of 1.60, which is better than stock 455 and bigger cid.


And, if the 4.25 stroke crank is used with the 6.625 rod - R/S of 1.55, really bad?



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Old 01-12-2022, 12:33 PM
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Quote:
Originally Posted by johnta1 View Post
It's the R/S ratio that affects the side loading. It increases with SMALLER R/S ratio. (R/S - Rod length to Stroke length)
Higher R/S ratio makes for higher RPM with less loading.


The R/S of a 455 with stock stroke of 4.21 is 1.57, pretty bad.
The R/S of a 455 with same stroke and 6.8" rod length is R/S of 1.62, still probably bad but better than 1.57.
The R/S of a 4.151 bore and 4.25 stroke with 6.8 rod length is R/S of 1.60, which is better than stock 455 and bigger cid.


And, if the 4.25 stroke crank is used with the 6.625 rod - R/S of 1.55, really bad?


Chevy 454 is 1.53. How bad are those?

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Old 01-12-2022, 12:36 PM
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Bad?



If it used the 6.8" rod it would be 1.7 which is close to our 400 engine.


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Old 01-12-2022, 12:45 PM
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In fact the stock BBC Rod to stroke ratio is worse then a stock Pontiac 455 in fact!

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